The source material seems more than a little fragmented, lots of different sites, many proper PDFs, some scanned image PDFs (only one or two, many PDFs are that large because they are very rich in images), some are broken up into awkward chapters with awkward names, and so on.

At least one of them (the CK-12 calculus textbook available at http://cafreetextbooks.ck12.org/math/CK12_Calculus.pdf ) uses bitmap graphics for all mathematics --- not even bothering to line things up on the baseline and using a mix of Computer Modern (in w/ text) and Times / Symbol (for labelling figures).

Absolutely wretched and along the lines of one of my worst experiences w/ a boss attempting to outsource composition work.